The treatment of injuries or diseases of hard tissues often requires surgical action. In general, bone fractures are usually treated with wires, nails, screws, and plates; joints are replaced by artificial endoprostheses; and, lost teeth are replaced by implants in the jaw. There are mainly three types of materials used for manufacturing dental/orthopedic implants: metals, polymers and ceramics.
Metallic implants are typically made of elemental metals such as titanium, tantalum, niobium, zirconium and related alloys, certain types of stainless steel, cobalt-chrome alloys. All show good mechanical strength and biocompatibility, but lack in their ability to form direct bonds to new formed bone tissue in the body. As a result, these metals rub against the bones into which they have been implanted, creating wear and tear that shortens implant lifetimes.
Polymeric implants can generally be divided into two categories: one is a biocompatible polymer that has biochemical and biomechanical properties suitable for load bearing orthopedic implants, such as polyetheretherketone (PEEK), ultra high molecular weight polyethylene (UHMWPE), polymethyl methacrylate (PMMA), and the like. These thermoplastic materials, however, have the same problems as biocompatible metallic materials. The other polymeric category is bioactive polymers that are generally used as a temporary scaffold for tissue engineering applications. Examples of such bioactive polymers include polylactic acid (PLA), its co-polymer with glycolic acid (PLGA), polycaprolactone (PCL), polyhydroxyalkanoates (PHA) such as poly-3-hydroxybutyrate (P3HB), and the like. Due to their degradation in vivo, they have limited load carrying capability.
Ceramic implants such as bioglass and some calcium phosphates (CaPs), on the other hand, have an ability to form bonds with hard tissues such as bone tissue. There are several compounds in the calcium phosphate system with different degrees of biocompatibilities. The ceramic implants can be fabricated into 3D scaffolds for tissue engineering applications, but are limited in their application due to their brittleness. To overcome the brittle behavior of ceramics, toughened ceramics such as zirconia and related materials can be used. Still, such toughened ceramic implants are only biocompatible, not bioactive.
One particular CaP material is a hydroxyapatite (HA) compound that is similar to bone minerals, and has received approval by the FDA for many applications. Other CaP materials include, for example, tricalcium phosphate (TCP) and the like.
In certain cases, the CaP materials can be combined with metallic, polymer, or ceramic implants and used as coatings. However, there are still concerns with such coated CaP implants that are related to the chemical composition itself, and its crystallinity, biodegradability and useful thicknesses. These are of particular concern in the manufacturing since, in order to achieve the desirable crystallinity and biodegradability, the relevant manufacturing process may take such long times that is not commercially viable.
Another concern is the ability to manufacture implants that are suitable for extended use in a human body; for example, a CaP coating thickness on an implant of about 100 μm or greater can introduce fatigue under tensile loading. Moreover, it has been found that the residual stress increases with thickness, and its energy release may promote interfacial debonding.
In a past biomimetic coating technique, Ca2+ and PO43−/HPO42− in solution randomly form amorphous calcium phosphate (ACP) precursors that are capable of binding to active medical device surface as nucleation sites. However, the ACP precursors are very unstable and subsequently undergo rapid phase transformation into other calcium phosphate (CaP) materials, such as octacalcium phosphate (OCP) and hydroxyapatite (HA) with adsorption of extra Ca2+ and PO43−/HPO42− ions from solution. Another important and serious drawback of this biomimetic coating technique that the process requires up to 3 weeks in order to form uniform coatings; in addition to the extremely long processing time, there is a high risk of contamination during the coating step. In addition, this biomimetic coating technique usually produces a thick coating, which has a tendency to crack. In one attempt to shorten the coating time, the ionic strength of SBF was purposely increased to intensify the CaP formation rate. However, such intensification of the CaP precipitation led to less similarity of CaP to natural bone and to thicker coatings, as well as the non-desirable formation of loose particles.
As such, it would be advantageous to achieve thickness reduction for CaP implant materials, at least in part, because: (1) less material would be necessary, (2) there would be reduced residual stress, (3) better adhesion of the coating could be achieved, (4) there would be less time required to apply any coating of desired thickness, and (5) inclusion of silica ions in the form of nanocrystalline (colloidal) oxide to further enhance both bioactivity and adherence to the substrate.
It would be useful to have thin calcium phosphate coatings of about 10 μm or less can improve the bone response in orthopedic and dental implants, showing better adhesion to a variety of substrates and greater stability in the biological environment.